The present invention relates to single-phase AC induction motors, and in particular to a solid-state motor start circuit that controls flow of AC current to the motor auxiliary or start winding when the motor is started up.
At start up, AC single-phase induction motors require some sort of starting mechanism to rotate the magnetic field of the field windings, so as to generate sufficient torque to start the rotor. The starting mechanism enables the rotor to overcome the static forces associate with accelerating the rotor and its associated load. Different motors require different amount of additional torque at start up. Also, the amount of auxiliary current required depends on initial load conditions, and on the quality of the AC power.
The typical AC induction motor armature is equipped with two sets of windings, namely, one or more main or run windings for driving the motor at normal operating speed, and an auxiliary or start winding to generate the required starting torque. In order to provide the necessary rotating magnetic field for start-up, a phase shift device such as a capacitor is connected in series with the start winding. During start-up, both the run winding(s) and the auxiliary or start winding(s) are energized to bring the motor up to a sufficient operating speed. At that point, the start or auxiliary winding either drops out of circuit so that the motor operates on the run windings alone, or can be connected to a run capacitor but cut off from the start capacitor. In the event that a heavy load is encountered, and the motor rpm drops below its design operating speed or stalls, the auxiliary winding can be cut back in to increase motor torque, and overcome the increased load.
In most AC induction motors, the structure of the auxiliary winding is such that sustained connection to the AC line voltage could cause overheating and damage. For capacitor-type motors, the start capacitor can also suffer damage from sustained connection. Therefore, the start winding has to both connect and disconnect at the proper times, at start-up and afterwards.
Because of the relatively short operating life of centrifugal switches and other electromechanical devices, current and voltage sensing circuits have been employed to control auxiliary or start current. These can include a reed-switch/triac combination, as described in Fink et al. U.S. Pat. No. 3,766,457, or a current-sense-resistor based circuit as described in Lewus U.S. Pat. No. 3,916,275. Another solid-state motor start circuit is described in Kadah U.S. Pat. No. 4,820,964, in which a solid-state switch, such as a triac, controls the start current, and in which the switch is gated by a photosensitive element. In Kadai et al. U.S. Pat. No. 5,589,753, a start circuit for a single-phase AC induction motor uses a triac in series with the motor auxiliary winding, and which turns on in response to an increase in the time rate of change of auxiliary voltage.
A few simplified start circuits have been proposed but each of these has to be tailored to the motor it is associated with. These typically act to shut the start current off after the applied voltage, which is low during surge or inrush conditions at start-up, rises to some fixed voltage level. In one version, the AC voltage seen at the start capacitor is integrated, and this gates off a triac or similar switch when the voltage reaches a predetermined level. A timer may be also associated with this circuit to shut the start circuit off after some predetermined time has elapsed, for example, 300 milliseconds. Unfortunately, on many occasions, the quality of the input power may be poor, i.e., during "brownouts," in which the input voltage brings the motor up to speed very slowly. Also, the load conditions on the same motor may vary from one occasion to another. These start circuits have to be designed for worst-case conditions, however, and this means that for most other conditions more start current is applied to the auxiliary winding than is necessary, and can stress the motor.